1. Field of the Invention
This invention is generally related to an image forming apparatus, such as a printer, a copying machine and a facsimile apparatus. More specifically, this invention is directed to a device which prevents ions emitted by a corona discharge device from recharging exposed portions of the photosensitive drum of the image forming apparatus.
2. Description of the Related Art
Conventionally, a so-called scorotron-type charging device using a corona discharge is used as a charging device for an electrophotographic image forming apparatus. The structure of such a scorotron-type charging device 101 is schematically shown in FIG. 8. The charging device 101 is a scorotron having a corona wire 103, which is formed of a fine tungsten wire or the like. The corona wire 103 is stretched to be parallel to an axis of a cylindrical photosensitive drum 102. A grid electrode 104 for controlling the charge is positioned at a predetermined distance from the surface of the photosensitive drum 102. By applying a high voltage, typically several thousand volts, to the charging device 101, ions (positive charges) due to corona discharge are emitted from the corona wire 103 to the photosensitive drum 102. The surface of the photosensitive drum 102 is thus charged by the ions to a positive potential of around 800 V. At this time, the ions are controlled so they uniformly charge the surface of the photosensitive drum 102 by applying a voltage of several hundred volts to the grid electrode 104. The corona wire 103 is surrounded by a charger case 105, which opens toward the photosensitive drum 102.
A laser beam 106, which is modulated corresponding to printing data, is directed against or focussed on the photosensitive drum 102 having been charged as described above. The laser beam 106 discharges only those portions of the photosensitive drum 102 which are irradiated by the laser beam when it is modulated to be in a discharge state (i.e. on). The surface potential of the photosensitive drum 102 at the discharged portions is thus discharged from the original 800 V surface potential to around 100 V. If positively charged toner is then supplied to the photosensitive drum 102, the toner attaches only to the discharged portions of the photosensitive drum 102 where the surface potential is lower than the surrounding charged portions of the photosensitive drum 102, i.e., the portions irradiated by the laser beam 106 when it is on. Thus, a developed electrostatic image is formed on the surface of the photosensitive drum 102. The image is transferred to recording paper by a known transfer device. The transferred image is then fixed to the recording paper by a known fixing device.
However, in the known image forming apparatus as described above, it sometimes occurs that positive charges (ions) produced by the corona device 101 improperly fly to discharged portions where the potential has been lowered after discharging by irradiation of the laser beam 106, thus raising the potential at such recharged portions. In this case, a recharged portion to which the toner should naturally attach to form the developed image instead repels the toner similarly to the charged portions, thus preventing the toner from attaching to the recharged portions of the photosensitive drum 102. Accordingly, while such recharged portions should be used to form developed portions of the image, they instead form blank portions of the image, or fail to have sufficient density, thus severely reducing the quality of the formed image.
To prevent such negative effects, increasing the distance between the position of the charging device 101 and the position where the laser beam 106 is directed against or focussed on the photosensitive drum 102 (i.e. the exposure position where the latent image is formed on the photosensitive drum 102) is considered to be effective when the distance is such the ions produced by the charging device 101 cannot reach the exposure position. However, to accomplish this increase in distance, the image forming apparatus becomes larger and difficulties occur in constructing the apparatus.